Which times on a digital clock have a line of symmetry? Which look
the same upside-down? You might like to try this investigation and
This investigation explores using different shapes as the hands of
the clock. What things occur as the the hands move.
How many days are there between February 25th 2000 and March 11th?
Published March 2006,April 2006,February 2011.
If you have ever travelled abroad, then you might have had to
change your watch to coincide with the local time at your
destination. Have you ever wondered who decided where the time
should change? How did they decide by how much it would differ? On
a recent trip to the Royal
Observatory in Greenwich , London, I discovered the answers to
these questions with the help of Graham Dolan, the Senior Education
Officer. You might have heard of the Royal Observatory, or of
Greenwich itself. If you have, it could be because you are used to
the expression "Greenwich Mean Time", or maybe you've heard of the
Prime Meridian which is located at Greenwich.
The NRICH article called
Coordinates explains the first part of the story. Eratosthenes,
a Greek scholar, was the first person to make a map of the world
based on a system of grid lines. These imaginary lines are of two
types: lines of latitude are "horizontal" and run parallel to the
equator; lines of longitude are "vertical" and run from pole to
pole. Lines of longitude are also known as meridians and the
meridian passing through a certain place is called the local
In the early 18th century, there was still not a good way of
knowing the position of a ship at sea in terms of its longitude.
People had believed that the moon would be a good way to measure
position on the earth and the observatory at Greenwich was built to
try and prove this theory. In 1766, the Astronomer Royal, Nevil
Maskelyne, worked out where the moon would be throughout 1767 if
you were standing on the earth. Maskelyne calculated the angular
position of the moon in relation to several close bright stars
every three hours, based on the local time at Greenwich. Sailors
could use an instrument called a marine sextant along with the
figures that Maskelyne published to find out their longitude
compared with Greenwich.
The Prime Meridian at Greenwich therefore became zero degrees
longitude and the time at Greenwich became known as Greenwich Mean
Time. It is from this benchmark that times across the world are
calculated. When it is 12 noon in Greenwich, it is 12 midnight on
exactly the other side of the world, 180 degrees longitude. The
rest of the world is divided up into 24 time zones - you could work
out how many degrees wide they are! As you go from west to east,
you move ahead one hour for each zone. Some countries are so large
that they span more than one time zone. In the USA, for example,
you would have to change your watch if you travel far enough.
You might be interested to know that the Prime Meridian passes
very close to Cambridge where NRICH is based. In fact, it runs
through the villages of Meldreth and Hardwick, just west of
Cambridge and that is where The Meridian School in Royston gets its
name. Why not find out how far your school is from the Prime
Meridian? You might be able to find its positionand so stand with
one foot in the east and one in the west!
If you would like to find out more about the Greenwich Meridian,
or about modern tracking systems, then I would recommend Graham
Dolan's book, "On the Line The Story of the Greenwich Meridian",
published by the National Maritime Museum, Greenwich. If you are
lucky enough to live near London, then why not try and visit the Museum and the Royal Observatory ?
You might even be able to persuade your teacher to arrange a